Thornton High School near Denver is under lockdown Friday afternoon as police investigate reports of a gunman near the campus on the last day of final exams.

Thornton Police Department Officer Matt Barnes said the first call about a gunman was received at 11:50 a.m. MDT (1:50 p.m. EDT), Fox 31 in Denver reported. A gunman has not been found as of this article’s publication.

“We’re still methodically searching the building. We’ve got quite a few teams in the school,” the Denver Post quoted Barnes as saying. “We’re developing a plan to get students home.”

Police ordered students to evacuate Thornton High School, and the Denver-area school issued an automated warning through its public address system, Thornton student Ezequiel Cordova told the Associated Press. Police believe all students at the school are safe, AP reported.

The high school is not the only school affected. On lockouts are Thornton Elementary and Middle schools, the Coronado Hills Elementary School, the International School at Thornton Middle School and the McElwain Elementary School, according to 9News in Denver. All these schools are in Adams County. Under a lockout, visitors are not allowed in a school but classes are still conducted.

Thornton High School was originally supposed to have let out students at 12:30 p.m. because of Friday being the last day of exams, 9 News reported.